For more satisfactory safety and lighter weight of automobiles, automotive steel sheets should have higher and higher strengths in recent years. Independently, such steel sheets should have excellent weldability upon manufacture of automotive steel parts. Demands are therefore made to provide steel sheets having both high strengths and excellent weldability. For allowing steel sheets to have higher strengths, increase in amounts of alloy compositions is generally performed. However, the increase in amounts of alloy compositions often causes the steel sheets to have inferior weldability.
For ensuring excellent weldability, it is preferred to allow a steel sheet to be a low-alloy steel (to reduce amounts of alloy compositions). For ensuring both excellent weldability and a high strength, steel sheets are allowed to have a martensite single-phase structure as the structure so as to be high-strength steel sheets (particularly steel sheets having tensile strengths of 1180 MPa or more) with low-alloy compositions.
Some high-strength steel sheets are subjected to seam welding upon processing into part shapes. The seam welding is a kind of resistance welding, and exemplary resistance welding techniques further include spot welding, in addition to seam welding. In the spot welding, welding is performed while sandwiching a steel sheet by electrodes at one point, and the work is air-cooled immediately after heat input. In contrast, in the seam welding, welding is performed in the form of a line while pinching a steel sheet by electrode wheels, in which a weld bead formed in the early stages of welding is affected by heat input of another weld bead being subsequently welded. The seam welding therefore differs in heat input process from the spot welding. The seam welding also differs in welding conditions, in which welding is performed continuously and this causes a shunt current to an already-formed nugget.
As is described above, a steel sheet preferably has a low-alloy composition for ensuring excellent weldability. However, even such a martensite steel sheet (high-strength steel sheet), when subjected to seam welding, suffers from an insufficient peel strength of a weld bead (hereinafter also referred to as a “seam weld bead”). To avoid this problem, the high-strength steel sheet should give seam weld beads having a higher peel strength. In addition, the high-strength steel sheet desirably gives seam weld beads having satisfactory bending workability.
An exemplary technique relating to martensite steel sheets having low-alloy composition is as follows. Japanese Unexamined Patent Application Publication (JP-A) No. H07-197183 discloses a steel sheet having a martensite-based structure, in which Fe—C precipitates are controlled so as to avoid hydrogen embrittlement. This technique, however, never makes considerations about weldability (particularly properties of seam weld beads when subjected to seam welding).
An exemplary technique relating to resistance welding is as follows. U.S. Unexamined Patent Application Publication No. 2007/0269678 describes a technique of improving the bonding strength of weld beads by controlling the amount of Mn to be added. This technique, however, is not examined specifically on seam welding among the resistance welding techniques, and the chemical composition disclosed therein is probably not suitable for seam welding.
Japanese Unexamined Patent Application Publication (JP-A) No. 2002-363650 describes a technique for improving seam weldability by controlling a Si content. This technique makes a specific consideration on reduction in hardness of nuggets formed after seam welding, but fails to examine the peel strength and the workability of seam weld beads.